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Dan Schwarz
Structure and bonding in actinide complexes
Actinides and their binary compounds are of tremendous technological
importance. They find application as fuels for nuclear power production,
power supplies for deep-space exploration, and in nuclear weapons.
The storage and disposal of used nuclear fuels, the disposition of
facilities and wastes from nuclear weapons production sites, the transport
of actinides at environmental contamination sites, and the storage
lifetime of nuclear weapons pose a variety of problems that require
research so that we may properly handle the material or site of concern.
The
deceptively simple binary formula of the tetravalent actinide oxides,
AnO2, hides an incredibly complex structural nature, and
a tendency to form nonstoichiometric phases (AnO2±x).
For plutonium (Pu), it was a widely held view that compositions with
oxygen/plutonium ratios higher than 2.0 were not stable. This view
was challenged when the reaction of plutonium metal or oxide with
water vapor was shown to produce PuO2+x. This new material
was examined by X-ray absorption fine structure (XAFS) spectroscopy
and found to be comprised of a mixed-valent solid of Pu(IV)/Pu(V)
oxidation states with short “plutonyl-like” Pu=O
bonds of 1.85 angstroms (Å).
We are currently investigating similar behavior
in mixed-valent uranium oxides of general formula UO2+x. UO2 and U4O9
(UO2.25) have been synthesized and studied by XAFS, along with several
intermediate compositions of UO2+x. We have found that as x increases,
a new feature at 1.74 Å,
which has been assigned as U=O, also increases. An example of this new feature
can
be seen in data represented by the green line in the figure comparing
different UO2 samples.
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Extended X-ray absorption fine structure data comparing four
samples. Higher peaks indicate greater long-range order.
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These short contacts are not consistent with
what has been previously reported using neutron diffraction and pair
distribution function; however, these techniques look for long-range
order, and this new short contact is a small percentage and probably
highly disordered so it would not be observed by those techniques.
It is only by using XAFS, which looks at local environments, that
we have been able to observe the formation of this new type of bonding
in a uranium oxide material. We have established XAFS capabilities
at the Stanford Synchrotron Radiation Laboratory beamline 11-2 for
uranium and transuranic samples.
We are also investigating the extent
of covalent bonding in actinide complexes in a variety of compounds,
including the isostructural UCl6x- series (x = 1, 2, 3). Our results
challenge the dogma of actinide chemistry that 5f electrons don’t
contribute to the formation of covalent bonds. In fact, our data show
that UCl62- has about half the covalency found in transition metals with
contributions from both the 6d and 5f orbitals. We plan to extend
out analysis beyond chlorine (Cl) edges to include oxygen (O) and
nitrogen (N) edges, which would require ultra-high vacuum and a special
chamber. Improving our understanding of the extent of covalent bonding
can aid in ligand design for fuel separation and reprocessing.
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 Dan
Schwarz works in the Chemistry Division’s Inorganic, Isotope, and Actinide Chemistry Group (C-IIAC). He received his doctorate in 2003 from the University of Illinois—Urbana-Champaign under the direction of Tom Rauchfuss. The title of his thesis was “Tetrathiometallates in Synthesis and Catalysis.” Schwarz
began his Seaborg postdoctoral appointment in July 2007. His mentor
is David L. Clark of the Seaborg Institute.
 A view of beamline 11-2 at the Stanford Synchrotron Radiation Laboratory
where the uranium L-edge studies are performed. The yellow glow in the photo
comes from a tent around the room.
 The high-vacuum K-edge apparatus for X-ray absorption fine structure
studies at the Stanford Synchrotron Radiation Laboratory.
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